1. Field of the Invention
The invention disclosed in this specification relates to substances that can emit light by electric current excitation. In particular, the invention relates to substances that can emit light from a triplet excited state. In addition, the invention relates to light-emitting elements, display devices, electronic devices, light-emitting devices, and lighting devices in each of which any of the aforementioned substances is used.
2. Description of the Related Art
A light-emitting element includes a layer (a light-emitting layer) containing a light-emitting substance between a pair of electrodes (an anode and a cathode). It has been reported that a variety of organic compounds can be used as the light-emitting substance.
It is said that light emission mechanism of a light-emitting element is as follows: when voltage is applied between a pair of electrodes with a light-emitting layer interposed therebetween, electrons injected from a cathode and holes injected from an anode are recombined in the light emission center of the light-emitting layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons relax to a ground state. Singlet excitation and triplet excitation are known as excited states, and light emission can probably be achieved through either of the excited states.
In such a light-emitting element, since more excitons are generated in a triplet excited state than in a singlet excited state, emission efficiency of the light-emitting element can be increased by using a material that can emit light from a triplet excited state (a phosphorescent material). Therefore, numerous attempts to use a phosphorescent material as a light-emitting substance have been made.
A metal complex where iridium (Ir) is the central metal (hereinafter, referred to as an Ir complex) is a typical phosphorescent material which emits green to blue light (for example, see Patent Document 1). Disclosed in Patent Document 1 is an Ir complex where a triazole derivative is a ligand.